Neurohumoral inflammatory mediators promote calcium entry through store operated calcium entry channels, and the resulting rise in cytosolic calcium disrupts the endothelial cell barrier and causes pulmonary edema. Canonical transient receptor potential proteins 1 and 4 contribute subunits that form the endothelial cell store operated calcium entry channel (TRPC1/4). Orai1 interacts with the this channel, and controls TRPC1/4 ion selectivity, as determined by whole cell electrophysiology. Our work has recently shown that in the absence of Orai1, more sodium and less calcium permeates through TRPC1/4. Because calcium is a principal second messenger that activates intracellular enzymes and regulatory proteins, considerable work has focused on the determinants of calcium influx. However, these new results suggest a potentially important role for sodium influx through TRPC1/4. We therefore measured the cytosolic calcium and sodium responses following activation of store operated calcium entry channels. Similar to our patch clamp results, we found that thapsigargin increases cytosolic sodium. Orai1 silencing substantially increases this cytosolic sodium response, but has a relatively small impact on the global cytosolic calcium response. Orai1 silencing prevented thapsigargin-induced gap formation. We interpreted these data to mean that elevated cytosolic sodium is a critical determinant of cellular movement. This idea was tested further in preliminary studies using migration and angiogenesis assays, where we observed that increased cytosolic sodium impairs cellular motility and angiogenesis. Hence, we now test the HYPOTHESIS that Orai1 interacts with TRPC1/4 channels and modulates the channel's calcium and sodium selectivity important for control of endothelial cell barrier function, migration and angiogenesis. Specific aims test the related hypotheses that: (1) Orai1 interaction with the TRPC1/4 channel modulates the magnitude and spatial spread ofthe cytosolic sodium and calcium signals; and (2) sodium influx through the Orai1-TRPC1/4 channel strengthens the endothelial cell barrier and reduces cell migration and angiogenesis. RELEVANCE (See instructions): Pulmonary endothelial cell permeability increases during inflammation, and is an important cause of patient morbidity and mortality. Here, we identify an endothelial cell calcium channel that contributes to permeability, and may therefore represent a novel anti-inflammatory target.